The laser printer is used as a high-speed terminal device or the like for printing out an output of a computer. FIG. 1 shows an example of optical system of the laser printer. With reference to FIG. 1, the light emitted by a semiconductor laser 1 is collimated by a collimator lens 2 into a beam of parallel rays incident on a polygonal mirror 3. Semiconductor lasers, which are of small size, have been introduced into use in recent years in place of He-Ne gas lasers and the like and are generally about 780 nm in the oscillation wavelength.
The polygonal mirror 3 is rotated at a high speed by an unillustrated motor. The beam reflected from the mirror 3 is subjected to scanning by the rotation and impinges on a photosensitive drum 6 via an image forming lens 4 and a mirror 5. Because the semiconductor laser 1 is turned on and off in response to an output from the computer, an image of electric charges is formed on the drum 6 in conformity with the on-off action of the laser 1. The charge image on the drum 6 is treated by a known electrophotographic process.
The characteristics required of the mirror for use in such laser printers are high reflectance, high durability and ease of fabrication.
The high reflectance reduces the optical loss to be involved in the optical system, permitting use of a laser of lower output and leading to a cost reduction. The reflectance must be sufficiently high in the near infrared wavelength region which includes the oscillation wavelength of the laser, i.e. about 780 nm in the case of the semiconductor laser.
With laser printers, the dust adhering to the surfaces of the mirrors, especially to the surface of the polygonal mirror, produces an appreciable influence on the copy images, so that there arises a need to clean the mirror surface. In this respect, the durability of the mirror is of importance.
An ease of mirror fabrication of course would provide greater economy.
The mirrors heretofore used for laser printers are commonly those prepared by depositing Al on a substrate by vacuum evaporation, because Al mirrors are generally superior in reflectance, durability and ease of fabrication.
When compared only in reflectance, Au, Cu and Ag have a higher reflectance, but Au exhibits poor adhesion on vacuum evaporation and is low in durability. Cu and Ag undergo a marked change with time due to oxidation or the like.
FIG. 2, showing the reflectance-wavelength characteristics of Al, reveals that this metal has a decreased reflectance at about 780 nm which is the oscillation wavelength of the semiconductor laser. FIG. 2 also shows the reflectance characteristics of Au and Cu which are superior to Al in reflectance. Al is approximately 15% lower than Au and Cu in reflectance at around 780 nm.
It will therefore be understood that although satisfactory in various characteristics requirements on the average, the Al mirror is slightly inferior in reflectance for use with semiconductor lasers.
As a method of giving an improved reflectance to Al mirrors, it appears useful to coat the Al mirror with a reflection increasing film. Since this film is designed to afford an increased reflectance by film-with-film interference, the film must be formed with an accurately controlled thickness. When the polygonal mirror for laser printers is to be coated with a reflection increasing film, extreme difficulties ar encountered in forming a thin film of uniform thickness on each side of the mirror which is in the form of a polygonal prism.